The 23-year-old moves to Old Trafford on a five-year contract with the option to extend for a further year.
Van de Beek, who is currently on international duty with Netherlands, is United’s first summer signing.
He was left out of Ajax’s squad over the weekend for friendly games against Eintracht Frankfurt and Union Berlin once United firmed up their interest.
Speaking to Man Utd's official website, the Dutchman said: “I cannot begin to explain how incredible an opportunity it is to join a club with such an amazing history.
“I would like to thank everyone at Ajax. I grew up there and I will always have a special bond with the club.
“I am now ready to take the next step in my career and perform at the highest level and there is no higher standard than Manchester United.
“Everyone has told me how amazing the Old Trafford atmosphere is and I am looking forward to experiencing that, once it is safe for the fans to return.
“This team has some of the best midfielders in the world and I know that I can learn from them and also bring my own strengths to the group.
“Having spoken to the manager about his vision for this team, the direction that the club is going in is hugely exciting and I cannot wait to be a part of that.”
Van de Beek made 175 appearances at the Johan Cruijff Arena after coming through the Ajax youth ranks and scored 41 goals.
He was part of the Ajax side that won the Eredivisie title and reached the Champions League finals in the 2018/2019 season.
Van de Beek could make his debut for the Red Devils against Crystal Palace on September 19.
